Arithmetical operations of this kind occur notably in digital signal processors and serve, for example, to realize scalar products as well as time-discrete correlation, filter and convolution operations. In order to enable fast execution of such operations there is customarily provided a multiplier device which successively receives both data words of each pair and whose output is connected to one input of the add/subtract device, the other input of this device being connected to the output of the device via a so-called accumulator register. When the device is suitably constructed, a multiplication/accumulation step as well as the required data word transfers can be executed within a single instruction cycle of the processor. The formation of the overall sum then requires merely a number of instruction cycles which is equal to the number of data word pairs plus some preparatory instructions.
Many fields of digital signal processing application also involve summing of products of a concatenation of data word pairs where one data word of each pair can assume only one of the two values +1 or -1. Examples of such fields of application are scalar products as well as time-discrete correlation, filter and convolution operations with binary bipolar coefficients as they occur in signal processing whenever binary signals are used.
The formation of such a sum by means of a multiplier device with a subsequent add/subtract device leads to uneconomical use of the multiplier, because it performs merely multiplications by the factors +1 or -1 on the respective other data words of the pairs, i.e. it realizes merely a conditional change of sign of these data words.
U.S. Pat. No. 4,849,922 discloses an arrangement of the kind set forth in which the multiplier is realised by arithmetically shifted additions or subtractions. The construction of this known device is somewhat simpler than that of a parallel multiplier device for data words with many positions, but such an arrangement is again uneconomically used for multiplications by merely the factors +1 and -1.